Abstract
Abstract. We present a parameterization of cirrus cloud formation that computes the ice crystal number and size distribution under the presence of homogeneous and heterogeneous freezing. The parameterization is very simple to apply and is derived from the analytical solution of the cloud parcel equations, assuming that the ice nuclei population is monodisperse and chemically homogeneous. In addition to the ice distribution, an analytical expression is provided for the limiting ice nuclei number concentration that suppresses ice formation from homogeneous freezing. The parameterization is evaluated against a detailed numerical parcel model, and reproduces numerical simulations over a wide range of conditions with an average error of 6±33%. The parameterization also compares favorably against other formulations that require some form of numerical integration.
Highlights
Cirrus clouds are key components of climate and can have a major impact on its radiative balance (Liou, 1986; Hartmann et al, 2001; Lohmann et al, 2004)
We develop a novel method to account for the growth of the heterogeneously frozen ice crystals and incorporate their effect within the physicallybased homogeneous nucleation framework of Barahona and Nenes (2008, hereinafter BN08)
The performance of the parameterization for pure homogeneous freezing conditions was discussed in BN08, so the evaluation here is focused on the competition between homogeneous and heterogeneous freezing
Summary
Cirrus can form by either homogeneous or heterogeneous freezing. Homogeneous freezing occurs spontaneously from temperature and density fluctuations within a supercooled liquid phase (Pruppacher and Klett, 1997); whether it happens largely depends on the temperature, droplet volume, and water activity within the liquid phase (Chen et al, 2000; Koop et al, 2000; Cziczo and Abbatt, 2001; Lin et al, 2002). IN and supercooled droplets may be considered separate populations that interact through the gas phase, but undergo heterogeneous or homogeneous freezing, respectively, at different stages during cloud formation (DeMott et al, 1997) This distinction may become less clear for heterogeneous freezing in the immersion mode for low concentration of immersed solid, or, if the solid is not an efficient ice nuclei (i.e. Sh is high, close to the value for homogeneous freezing) (Khvorostyanov and Curry, 2004; Marcolli et al, 2007); we will assume for the rest of this study that we are far from such conditions, given that their importance for the atmosphere is not clear. The first comprehensive physically-based parameterization of cirrus formation was presented by Karcher et al (2006) in which homogeneous and heterogeneous nucleation are considered In this scheme, the competition between different freezing mechanisms and particle types is resolved by using numerical integration to calculate the size and number of ice crystals at different stages of the parcel ascent. The extension of the parameterization to polydisperse IN (in size and chemical composition) will be presented in a companion study
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